WASHINGTON – Nearly five-dozen members of Congress have written Agriculture Secretary Tom Vilsack urging stricter enforcement of horse-protection laws — including regulations against foreign substances — at the Tennessee Walking Horse National Celebration under way in Shelbyville.

The 59 House members, mainly Democrats, called for USDA to "undertake a substantial and active on-the-ground role to ensure strict enforcement of the Horse Protection Act."

Rep. Ed Whitfield, R-Ky., released the letter, sent earlier this month, as he continues to fight for adoption of the Prevent All Soring Tactics Act, which would significantly boost USDA's ability to enforce the 1970 law while also outlawing special pads, chains and other "action devices" associated with the practice.

The Celebration started Wednesday and runs through Aug. 30.

Widely seen as cruel, soring involves the intentional infliction of pain on a horse's lower front legs and hooves so that they recoil in agony when they touch the ground and achieve the higher-stepping "Big Lick" gait.

The lawmakers urged the department not to rely on industry-hired inspectors but to have its own personnel "aggressively inspect" horses in each class, both before and after shows.

The Celebration's "open collaboration" with groups that oppose the PAST Act, the letter says, indicates the show itself "is not committed to ending soring practices."

The letter added, "In our view, soring will continue unabated unless the changes contemplated by the PAST ACT are implemented."

In a statement accompanying the letter, Whitfield said 67 percent of horses tested at the 2013 Celebration — as well as 76 percent of those tested the year before — showed evidence of foreign substances used to mask soring.

Mike Inman, chief executive officer of the celebration, said in a statement that attacking the integrity of the inspectors was an attack on USDA itself, since industry-employed inspectors — known as "designated qualified persons (DQPs)" — receive their training from the Animal and Plant Health Inspection Service, a division of the federal agency.

"These people are trained, tested, and certified annually — regardless of how many years they have been doing the job — by the USDA. If the USDA did not feel they were competent, they wouldn't hold these positions," Inman said.

The Celebration official also cited the industry's establishment this year of a Veterinary Advisory Council to conduct blood tests and X-rays as evidence of its commitment to end soring.

PAST Act supporters and other critics respond that DQPs, despite USDA training, are hired and paid for by the industry itself, leading to an inherent conflict of interest and a reluctance to issue Horse Protection Act violations.

They argue that the new Veterinary Advisory Council, whose members are also industry-paid, suffers from the same conflict of interest.

Meanwhile, the Humane Society of the United States is urging the USDA to prohibit owners suspended for soring allegations from transferring ownership of walking horses to others so the animals can continue to compete.

The group said it had uncovered instances where ownership of as many as 10 horses had been transferred to another person so they could continue to compete.